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Abstract

A new compact infrared spectrometer without any mechanical moving elements has been designed and constructed using a two-dimensional InGaAs array detector and 10 sub-gratings. The instrument is compact, with a double-folded optical path configuration. The spectra are densely 10-folded to achieve 0.07-nm spectral resolution and a 2-ms data acquisition time in the 1450- to 1650-nm wavelength region, making the instrument useful for real-time spectroscopic data analyses in optical communication and many other fields.

Figures (9)

Schematic of light diffraction at the grating surface where both the incident light at angle i and diffracted light at angle θ are located on the same side of the surface normal n̂ according to Eq. (1).

Schematic of the spectrometer, showing the double-folded light path. (a). Optical elements are shown in the x-y plane. The light enters through the slit, with its path folded by plane mirror M1. The spherical mirror M2 reflects and collimates the light onto grating G, which consists of 10 sub-gratings set at slightly different angles. M3 and M4 are cylindrical mirrors. M3 has a focal length of 700 mm, with its cylindrical axis aligned along the z direction to focus the diffracted light onto focal plane P of detector D along the spectral (x) direction. M4 has a focal length of 375 mm, and is aligned with its cylindrical axis along the x direction to fold the light path and direct the light diffracted from the 10 sub-gratings onto the focal plane of the detector along the z direction. (b) and (c). Optical elements showing the folded light path as seen in the y-z plane.

10 sub-gratings, with a size of 100 mm×10 mm for each sub-grating, are set at slightly different angles to diffract light in 10 wavelength windows in the 1450- to 1650-nm wavelength region. The sequence of these windows is as follows: 1450–1470 nm, 1470–1490 nm, 1490–1510 nm, 1510–1530 nm, 1530–1550 nm, 1550–1570 nm, 1570–1590 nm, 1590–1610 nm, 1610–1630 nm, and 1630–1650 nm. The integrated grating has a size of ~100 mm×100 mm. Each sub-grating is masked by a long, rectangular window to block the potential stray light for cross-talk between neighboring gratings. The dispersive color spectra shown on each sub-grating arise from diffraction of visible light in the lab room.

The image of 10 sub-wavelength windows can be clearly seen and monitored using an Ultra-Wideband Source (UWS-1000) on the focal plane of the two-dimensional InGaAs infrared array detector (XenICs-XEVA-FPA-320) [12, 13].

The schematic diagram of the electron beam evaporation system integrated with the broadband spectrometer. The collimated probe beam of the light source in the 1200–2000 nm wavelength range entered the vacuum chamber and passed through the sample. The transmitted light was coupled into an optical fiber that was connected to the entry slit of the spectrometer. The transmission spectrum of the sample was measured in situ with high speed and high spectral resolution. After analysis of the transmittance spectrum, the thin film growth process was controlled precisely by the computer to produce a device which met the initially specified spectral requirements.

(a). The calculated and in situ measured transmittance spectra of samples with layer numbers of 25, 26 and 27. The demonstrated sample has 27 layers deposited on a Si substrate which was polished on both sides prior to layer deposition. The film structure of this substrate is Si/9L/[1H1L]4/2H/[1L1H]4/air, where L and H indicate the low and high refractive index film materials (SiO2 and Ta2O5, respectively). The optical layer thickness is designed to be equal to a quarter of the wavelength at 1540nm. (b). The calculated and in situ measured transmittance spectra of samples with a fractional layer thickness. The peak wavelength of the spectrum is shifted to ~1542.7 nm and ~1535.7 nm for fractional layer numbers of 25.2 (0.2 SiO2 layer) and 26.7 (0.7 Ta2O5 layer), respectively.